Have you considered using a generator instead? 15 minutes should be plenty of time to fire up the generator and plug the UPS into that.
You should be able to measure the battery voltage with a cheap multimeter. If you do connect a larger battery, make sure you use thick wires and be prepared for things to blow up (literally - have copious amounts of water available to an assistant (although I very much doubt if the battery would explode) and wear safety goggles).
You could contact APC directly.